Rats dot their environment with tiny drops of urine, called
urine
marking, or scent marking. Urine marking is a form of chemical
communication. Urine is full of information about the rat who
produced it: its species, sex, age, reproductive status, sexual
availability, social status, individual identity, and current stress
level, as well as the age of the scent mark itself.

Urine marking serves several functions. It acts as a sexual
attractant and advertisement to rats of the opposite sex. Urine
marking may also serve as a habitat labeling system. Rats may urine
mark their environment in order to maintain an optimum odor field, a
sort of olfactory landscape, which may help them maintain a sense of
familiarity with an area. Urine marking of the environment may be
serve as an index of spatial knowledge.

We humans can barely perceive scent marks ourselves, because once
a scent mark dries it becomes invisible to us. We may be able to
detect some of them through touch, as a scent mark on a hard surface
may attract a thin layer of dust and may feel rough to the touch, but
this is a far cry from the rat's awareness of his scentscape.

Ultraviolet
light

Fluorescent black light

Technology comes to the rescue, however. Rat urine fluoresces yellow
under ultraviolet light. And urine isn't the only substance that
glows under a black light. Porphyrin, a
component of rat tears, glows bright pink or magenta under a black
light as well. Porphyrin and other secretions coat the eye, drain
down into the nose through a small duct and may appear around the
nostrils.

What other substances glow under a black
light?

Urine, porphyrin, semen, sperm and saliva fluoresce
under a blacklight.

Blood does not fluoresce under a blacklight: it looks
black. To glow blood needs to be treated with a chemical
developer such as Luminol, which makes the blood glow in
the dark briefly even without a blacklight. This glowing
effect lasts only a few seconds and is used to
investigate crime scenes (Penven 2005). Skin does not
fluoresce, but it looks purple under a black light.

With this knowledge in mind, I purchased a fluorescent black light
at my local hardware store. For those of you who want to try this
yourselves, make sure to get a fluorescent black light, the
kind with a fluorescence tube, and not an incandescent black
light, which is a regular light bulb with a special coating.
Incandescent bulbs aren't fluorescent and therefore don't work (I've
tried).

Scent marks

First, I turned the black light onto the rats' environment to see
if I could detect urine marks (a pursuit known as peeseekery).
I took one of the rats' metal shelves out of their cage and
photographed it under normal and black light:

Then I took a look at the rats' nestbox. I give the rats a new
cardboard nestbox every week, so the marks below were all made in
less than seven days:

Photograph of the rats' nestbox under
normal light (left) and ultraviolet light (right). The urine
marks are visible under normal light as a faint mottling,
but become more apparent under a fluorescent black
light.

As you can see from the marks on the nestbox, the rats prefer to
sit on the front left quadrant of the box (the other areas are
overhung with a shelf and ramp). From the photograph, it looks like
the rats deposit scent marks in several ways. They deposit a scent
mark as they step on the box, dragging their urogenital area over the
box's edge, thus depositing a small drip on the front panel and a
smear along the top. The smear points in the direction they were
moving at the time. Next, the rats deposit rounder, less smeared
scent marks on the top of the box itself. Lastly, the rats scent mark
the "sill" or "threshold" of the doorways as they go in and out.

Lastly, here are photographs of a plastic tool chest that the rats
climbed on:

Photographs of a plastic tool chest
under normal light (top), and of its front right corner
under black light (middle and bottom). Under ultraviolet
light the scent marks glow.

Rats under a black
light

Next, I looked at the rats themselves. I was startled to see that
all three rats had pink muzzles! This effect was most visible on
Cricket, who has the most white fur on his muzzle. The nose, lips,
chin, and whisker bed were colored pink:

As I examined the rats further I found that other body parts
looked pink or purple, too, particularly areas with thin, delicate
skin. The rats' ears looked purple or violet:

Snip's ears are a normal tan color
under normal light (left), but under a fluorescent black
light (right) they look violet.

The eyes and the skin around them looked dark purple:

Cricket's ears and eyelids, normally a
pale tan (left), look purple under a black light (right). A
little mucus around his nostrils looks clear and colorless
under normal light, but fluoresces dark pink under
ultraviolet light, probably from porphyrin.

The tops of the rats' paws looked pink under the black light, and
the soles of their feet looked purple (my own skin was purple
too).

Cricket's hind foot under normal light
(left) and under a fluorescent black light (right). The sole
is dark purple and the top of the paw is pink under
ultraviolet light.

Black light
photography

Equipment: I made the pictures on this page with a first
generation point-and-shoot digital camera (Kodak DC240), a
fluorescent black light from the hardware store, and some tinkering
with Adobe Photoshop. I am not a professional photographer by any
means, so I unfortunately do not have any high-tech filters, lenses,
or black lights. Such equipment would doubtless produce much better
photographs!

Constraints: My camera does not do well in low light
conditions, and the black light doesn't put out much visible light.
This had three important consequences: (1) I could only take close-up
photographs. Any pictures taken far away turned out completely black.
(2) Even with the close-ups, many of the photos turned out very dark
with glaring whites. These required tweaking to make them
presentable. (3) My camera had trouble focusing under low light,
making many of the black light photos blurry, so I shrank the images
and kept them small to increase their sharpness.

More black lights would help address these constraints, but at $17
per light I didn't want to buy more than one.

Photo tweaking: Using the advice of a photographer friend,
I used the following procedure in Adobe Photoshop to brighten the
dark areas of the photographs and tone down the whites. This
procedure is more subtle than tinkering with the overall brightness
and contrast of the image:

Copy the picture into a new file

Desaturate the color (e.g. turn it into a black and white
image)

Invert it so that light is dark and dark is light

Return to the original document and make a new layer

Paste desaturated, inverted image into the new layer

Change layer mode to "Overlay"

Fiddle with the opacity of the inverted layer if
necessary

This procedure can be repeated on the same picture to enhance
the effect

Ultraviolet light and safety: The ultraviolet light
spectrum is artibrarily divided into three ranges that have different
health risks: UVA (low risk), UVB (medium risk) and UVC (high risk).
Black lights emit UVA. Brief exposure to UVA isn't a big problem, but
prolonged UVA exposure can irritate or damage the eyes and skin. So,
if you try this yourself, don't stare right into the light and don't
expose your rats to the light for long.